Toph-An Arithmetic Problem

#include<stdio.h>

int main()
{
    int t,i,a,b,c,d;
    scanf("%d",&t);

    for(i=1;i<=t;i++)
    {
        scanf("%d%d%d%d",&a,&b,&c,&d);
        if(b-a==c-b)
        {
            printf("Case %d: %d\n",i,a+(c-b)*(d-1));
        }
        else
            printf("Case %d: Error\n",i);

    }
    return 0;
}